In most games I've played it usually makes life easier for the GM to use their own judgement. Just roll the dice for one attack first and see if it would fall within that range. If it is, then shield goes up. If not, then it doesn't and rinse and repeat. If the last one does hit but he doesn't particularly feel threatened by the damage potential then he might let that one hit pass. I don't want you to have to spend a week on fealing with one of my spells. Unless you're super picky about things, which never seems to work out in pbp to my experience, I'll trust you to make it make sense.